In the conventional wheeled chairs, the back frame and seat width thereof can be adjusted; for instance, the moving angle of the back frame of such chair is usually controlled by means of an electric motor, or an air-compressed cylinder, or a spring. However, a back frame controlled with a motor would involve a complex structure to cause a higher manufacturing cost. The air-compressed cylinder or the spring type of chair-back frame has a simple structure and a lower cost; however, such a chair-back frame should be operated by another person in case of a user suffering from S.G.I. because of the frame having no power structure. The seat width of such a chair can be adjusted by moving a sleeve pipe to change the space between the arm rest and the side frame thereof; the sleeve pipes are usually fixed by means of screw bolts. The seat depth of the aforesaid chair is unable to adjust for persons having different body sizes. Further, the conventional chair has its arm rests and side frames assembled together into a single piece, which has to be disassembled whenever a user moves from the chair to another seat or the like; therefore, it is deemed an inconvenient structure in terms operation by a user.